The Three Deserts
What makes a desert a desert? In the strictest terms, a desert is a
landscape where the annual rainfall is less than 10" a year. And that rainfall usually happens in the summer, not in the winter. This means deserts are one of the driest places on earth, and that the landforms, animals, and plants that reside there have adapted to this dry climate and have characteristics unique to desert life.
When most people think of the American Southwest, they think of prickly cactus, dry deserts, slow-crawling tortoise and howling coyotes in scrub covered hills. What we are learning in Camp Internet is that the Southwest is actually a complex region with lots of different geographic features. And while deserts ARE an important part of the Southwest, you might be interested to know that if we were talking ONLY about the Four Corners states of the American Southwest, then the desert region would only make up a third of its region. That means two-thirds of the Four Corners region is not a desert landscape when looking at a U.S. map.
But as we are learning in our expedition, the Southwest bioregion is larger than just what is called the Four Corners (Arizona, New Mexico, Colorado and Utah). The Southwest includes northern areas of Mexico, just like it did in the days before these countries were formed, plus areas of three other states in the United States. Keep in mind that Camp Internet is studying the Ancient Southwest at a scale that not only includes the Four Corner states, but also includes northern Mexico and Baja California, southern California, southern Nevada, and northwestern Texas. When you add all of those areas into the region, the desert areas suddenly become a much BIGGER area in our view of the Southwest. In fact, the desert amounts to more than half the region when seen on this larger scale - and - they are not just one desert but three different deserts.
The Three Deserts - Sonoran, Mojave, Chihuahuan
California and Nevada add the Mojave desert to the region; Mexico, western Texas, and southern New Mexico add the Chihuahuan desert to the region; and the tall saguaro cactus area most often thought of as the Southwest Desert turns out to be just the northern tip of the Sonoran Desert that extends down into Mexico and over to Baja California.
That makes THREE deserts, not just one, in the Southwest today.
REALLY Ancient Deserts
In the time of the dinosaurs, we are studying that portions of the Ancient Southwest were tropical moist forests and other parts were at times dry deserts. While the tropical moist forests no longer exist, the dry deserts have become a long-term part of the Southwest ecosystem. During the middle of the Jurassic Period when there were deserts in the Southwest, the dinosaurs who lived there were mostly small meat eaters because there was little green food to be found. They tended to be much smaller than their huge tropical plant-eating counterparts, and could survive on less water and less food. These characteristics are also true of animals that live in the southwest deserts today.
In Ancient Human Time
By the time the early peoples native to the Southwest settled in the region, the Southwest was much drier overall than it had been in Dinosaur Time, but it was not as dry as it is today. Pangaea had separated into continents that were positioned much as they are today. Gone were the miles and miles of shallow lakes and swamps; the Colorado Plateau had long ago lifted a mile into the sky, creating majestic rocky peaks and year round rivers. Gone were the dinosaur's lush ferns and broad-leafed forests. During this transition, the southwest was subtropical in many areas and supported parrots and other animals we tend to associate with a tropical landscape.
As the region grew increasingly dry, the rains came less often, and people found it very difficult to catch enough rainwater to make it through the year. Eventually many of the populations of native settlers moved closer and closer to the rivers, and they learned to construct irrigation canals to water their fields. But as it grew drier up on the Colorado Plateau, and in the surrounding dry canyons, the early peoples appear to have abandoned entire towns an cities due to long droughts as the region became more and more of a desert.
Where does the rain come from and why is it scarce?
Rain is moisture drawn up into clouds from the ocean. The clouds are then pushed inland by the wind and, when they meet tall mountains, they release their moisture. In the Southwest, the Sierra Nevada is blocking the eastern lands from the ocean-originated northwest rainwater which creates a rain shadow (dry climate right behind a tall mountain). By the time the clouds pass over the Sierras, they have no moisture left. That, combined with another evaporation process, renders the western Pacific clouds fairly useless as a source of rain in the desert Southwest. This other type of evaporation occurs when a bowl of desert land, surrounded by mountains, heats up in the sun each day. As its air heats up, it rises, and as it rises, it dries out any moisture left in the clouds passing overhead, and soon the clouds completely disappear.
The Southwest deserts do get rain in the summer months, and it travels up from the Gulf of California in clouds that do not have to encounter a mountain range as formidable as the Sierra Nevada to the west. These southern cloud formations still meet the same problem of evaporation. It is common to see rain clouds passing over the Southwest that have dark sheets of rain dropping from their under bellies, but no rain is reaching the ground. Where is this rain going ? It is being evaporated by the rising heat from the desert before it can reach the hungry parched soil. This is called a virga.
But when the cloud is big enough and holds enough moisture, even the heat rising from the desert cannot evaporate the falling rain fast enough. Then the desert welcomes its rare experience of heavy, powerful, pounding rain. And because the rain is so dense that it can make it to the ground with out evaporating, it is also so dense that it causes flash floods due to this immense volume of sudden down-pouring rainwater. When it rains, it pours !
We Love the Desert !
There are thousands of different kinds of plants and animals - and millions of people - who are happy to live in the unique desert climate of the Southwest. These two footed, four footed, no footed life forms have found ingenious ways to adapt to having less than 10" of rainfall a year, and then have learned to create a life for themselves that in many ways has not changed much since ancient times. Morning sunrises are still beautiful cloud streaked rainbows of color, mid day is still a good time to slow down and rest in the shade, evenings have a wonderful soft scented air, and the stars are still totally awesome in the wide open bowl of the heavens. Cacti are still slowly growing, taking hundreds of years to become high-rise apartments for tiny owls and noisy woodpeckers. Coyotes are still roaming the desert seeking shade under mesquite brush, or a jackrabbit for dinner. Roadrunners still scurry with feathers outstretched, hunting for a tasty lizard lunch. And eagles still soar in the sky, connecting the native people to their holy people above. There is something eternally ancient about the southwest desert - lets learn more!
· The Sonoran Desert Virtual Field Trip
· The Mojave Desert Virtual Field Trip
· Visit a Desert Oases
· The Chihuahuan Desert Virtual Field Trip
· Use the Desert Geography Glossary